The proposed experiments will explore the response of Entamoeba to cyst- inducing conditions at the levels of cellular ultrastructure, gene expression and protein synthesis. Knowledge of the requirements and responses of the encysting cell will provide information useful for designing approaches to interrupt this stage of the parasite life cycle, thereby lowering the number of infective forms of the parasite in the human population. E. invadens will initially be used as a model system for E. histolytica. Conditions of cyst formation will be optimized to obtain high yields of cysts from in vitro trophozoite cultures. Electron microscopy will be used to examine ultrastructural changes during encystation. Novel protein synthesis will be examined with metabolically labelled encysting cultures and antisera which recognize cyst-specific products. Genes expressed during encystation will be isolated and used to monitor the transcriptional response of parasites during encystment. Conditions which support encystment of E. histolytica will be sought, in order to develop an in vitro cyst-forming system for the human parasite.